Recent advances in solid-state organic lasers

Organic solid-state lasers are reviewed, with a special emphasis on works published during the last decade. Referring originally to dyes in solid-state polymeric matrices, organic lasers also include the rich family of organic semiconductors, paced by the rapid development of organic light emitting diodes. Organic lasers are broadly tunable coherent sources are potentially compact, convenient and manufactured at low-costs. In this review, we describe the basic photophysics of the materials used as gain media in organic lasers with a specific look at the distinctive feature of dyes and semiconductors. We also outline the laser architectures used in state-of-the-art organic lasers and the performances of these devices with regard to output power, lifetime, and beam quality. A survey of the recent trends in the field is given, highlighting the latest developments in terms of wavelength coverage, wavelength agility, efficiency and compactness, or towards integrated low-cost sources, with a special focus on the great challenges remaining for achieving direct electrical pumping. Finally, we discuss the very recent demonstration of new kinds of organic lasers based on polaritons or surface plasmons, which open new and very promising routes in the field of organic nanophotonics

The development of micropillars and two-dimensional nanocavities that incorporate an organic semiconductor thin film

Photonic crystals (PC) are periodic optical structures containing low and high refractive index layers that influence the propagation of electromagnetic waves. Photonic cavities can be created by inserting defects into a photonic crystal. Such structures have received significant attention due to their potential of confining light inside volumes (V) smaller than a cubic wavelength of light (λ/n)3 which can be used to enhance light-matter interaction. Cavity quality factor (Q) is useful for many applications that depend on the control of spontaneous emission from an emitter such quantum optical communication and low-threshold lasing. High Q/V values can also result in an enhancement of the radiative rates of an emitter placed on the surface of the cavity by means of the Purcell effect. This thesis concerns the fabrication and study of two types of optical cavity containing an organic-semiconductor material. The cavities explored are; (1) one-dimensional micropillar microcavities based on multilayer films of dielectric and organic materials, and (2) two-dimensional nanocavities defined into a photonic crystal slab. Firstly, light emission from a series of optical micropillar microcavities containing a thin fluorescent, red-emitting conjugated polymer film is investigated. The photoluminescence emission from the cavities is characterized using a Fourier imaging technique and it is shown that emission is quantised into a mode-structure resulting from both vertical and lateral optical confinement within the pillar. We show that optical-confinement effects result in a blue-shift of the fundamental mode as the pillar-diameter is reduced, with a model applied to describe the energy and distribution of the confined optical modes. Secondly, simulation, design, and analysis of two dimensional photonic crystal L3 nanocavities photonic crystal are presented. Nanocavities were then prepared from silicon nitride (SiN) as the cavity medium with the luminescence emitted from an organic material at red wavelengths that was coated on the cavity surface. To improve the quality factor of such structures, hole size, lattice constant and hole shift are systematically varied with their effect as cavity properties determined. Finite Difference Time Domain (FDTD) modelling is used to support the experimental work and predict the optimum design for such photonic crystal nanocavity devices. It is found that by fine-tuning the nearest neighbour air-holes close to the cavity edges, the cavity Q factor can be increased. As a result, we have obtained a single cavity mode having a Q-factor 938 at a wavelength of 652 nm. Here, the cavity Q factor then increases to 1100 at a wavelength of 687 nm as a result of coating a red-emitting conjugated polymer film onto the top surface of the nanocavity. We propose that this layer planarizes the dielectric surface and helps reduce optical losses as a result of scattering

Organic & Hybrid Photonic Crystals for Controlling Light-Matter Interaction Processes

Organic & Hybrid Photonic Crystals for Controlling Light-Matter Interaction Processe

Planar microcavities: Materials and processing for light control

Microcavities are a class of optical structures providing a versatile approach to engineering light matter interactions. In light of recent developments in materials processing technologies, in particular for organic and hybrid ones, and of the need for high efficiency optical systems, there has been extensive innovation and improvement in their design and realization leading to a multitude of structures and materials. Among these, closed multi-material microcavities or microresonators based on the effect of dielectric contrast have been attractive for their low losses, applicability in a wide spectral range, and customizability. High-dielectric contrast microcavities based on distributed Bragg reflectors have been adapted early on for their highly controlled fabrication and strong light confinement and proved to be essential in current technologies including lasers and light emitting diodes. In this review, we map their evolution from planar one-dimensional inorganic structures to more sophisticated designs incorporating various categories of organic and hybrid materials. Additionally, we provide an overview of state-of-the-art developments and limitations of this class of structures

Organic & Hybrid Photonic Crystals for Controlling Light-Matter Interaction Processes

Organic & Hybrid Photonic Crystals for Controlling Light-Matter Interaction Processe

Organic Materials for Photonics: Properties and Applications

Photonics will play a key-role for the future development of ICT and healthcare and organic semiconductors are promising candidates to fulfil the capacity of photonics and deliver on its promises. This “photonics revolution” relies on novel and more performing materials, tailored for the specific requirements of real-world applications, and on reliable and cheap technologies, which can attract investments to address the transition from academia to industry. In this dissertation, I will report my findings on conjugated polymers suitable for photonic applications and demonstrate their use into low-cost photonic structures, as proof of concept. The first part is dedicated to the study of an aggregation-induced emission polymer, whose fluorescence is enhanced in the aggregated solid-state thanks to the restrictions of intramolecular rotations in contrast to typical planar conjugated polymers. I will show its exceptional fundamental photophysical properties which enable the reduction of non-radiative pathways and makes it attractive for its use in organic light-emitting diodes. In the second part, I will present the application of conjugated polymers into flexible all-polymer microcavities fabricated through a low-cost process based on spin coating. The incorporation of functional defects in periodic dielectric structures with optical feedback will enable the change in the photonic density of states. I will report the investigation on photonic resonators embedding an aggregation-induced polymer emitting in the visible and a novel near-infrared oligomer, assessing high quality factors and tuning of their radiative rates to achieve low threshold optically pumped lasers. In the last part, I will show the infiltration of conjugated engineered materials into porous silicon microcavities to enable a novel class of photonically-enhanced chips for communications and sensing. A cheap electrochemical technique has been employed to fabricate one-dimensional resonators, which I characterized fully to demonstrate the variation of the photonic density of states and an efficient approach to novel hybrid photonic devices

Lasing and strong coupling in inorganic and organic photonic structures

Diese Arbeit beschäftigt sich mit der Untersuchung der starken Kopplung und Laseremission in Strukturen, die ZnO, ZnCdO oder organische Moleküle als aktives Material enthalten. Die ZnCdO basierten Vielfachquantengräben erreichen ihre Laserschwelle durch optische Ruckkopplung an streuenden Luftlöchern. Diese Emitter nennt man random laser. Die Dynamik ihrer Emission unter quasi-stationären Bedingungen ist der hier gezeigte Fokus. Hoch reproduzierbare Anregungen werden verwendet um sowohl die Dynamik eines einzelnen Beschusses aber auch die Unterschiede verschiedener Anregungen zu untersuchen. Die experimentellen Daten werden durch numerische Simulation qualitativ reproduziert und mit Methoden der Netzwerktheorie interpretiert. Die verbreitetere optische Rückkopplung durch einen Resonator wird in der Untersuchung des Moleküls L4P und seiner Spiro-derivate benutzt. Zwei identische Braggspiegel umschließen die aktive Schicht aus L4P-SP2, das in eine Polymermatrix eingebettet ist, eine Dicke von 12 Mikrometer hat und in einer einzelnen Mode lasert (schwache Kopplung). Durch Verringerung der aktiven Schicht auf die Hälfte der Resonanzwellenlänge wird das System in den Bereich der starken Kopplung gebracht. Eine Rabi-Aufspaltung von 90 meV wird zu beiden vibronischen Resonanzen beobachtet. Die energetische Position in Resonanz zu ZnO macht dieses Molekül zu einem guten Kandidaten für die Fertigung einer hybriden Mikrokavität im Bereich der starken Kopplung. Dies wurde in einer teilweise epitaktisch gewachsenen Mikrokavität angewandt, die aus einem ZnMgO basierten Braggspiegel und sechs Quantengräben besteht. Darauf folgt eine aufgeschleuderte Schicht von L4P in der Polymermatrix. Der Resonator wird mit einem dielektrischen Spiegel fertiggestellt. Tieftemperatur Reflektion zeigt eine deutlichen ausweichen und eine gleichverteilte Mischung der drei Resonanz im mittleren Polaritonzweig.This thesis presents the investigation of strong coupling and lasing in structures using ZnO, ZnCdO or organic molecules as active material. The ZnCdO based multi quantum well structures reach the lasing threshold by using scattering at air holes as the optical feedback. Such emitters are called random lasers. The dynamics of their emission under quasi-stationary condition is the point of interest presented. Highly reproducible excitations are used to investigate the single shot dynamics and their shot to shot differences. The experimental data is qualitatively reproduced by numerical simulation and interpreted by means of network theory. The more common optical feedback by a cavity is applied in the investigation of the molecule L4P and its spiro-derivatives. Using two identical SiO2/ZrO2 based Bragg reflectors surrounding an active layer of L4P-SP2 in a polymer matrix of approximately 12 microns thickness reached single mode lasing (weak coupling). Reducing the active layer thickness to half the resonance wavelength pushes the system into the strong coupling regime. Angular resolved reflectivity shows the anticrossing of the tuned cavity resonance to two vibronic transitions of the molecule. The Rabi-splitting to both vibronic resonances reaches around 90 meV. The energetic position in resonance to ZnO makes this molecule a promising candidate for a hybrid inorganic/organic microcavity in the strong coupling regime. This is used in a partially epitaxially grown microcavity composed of a ZnMgO based Bragg reflector (alternating layers of different Mg content) and six quantum wells. This is followed by a spincoated layer of L4P in a polymer matrix. The cavity is finished by a dielectric mirror. Low temperature reflectivity shows a clear anticrossing reaching an equal mixing of all resonances for the middle branch

New Polymer and Composite Structures for Photonic Applications

The focus of this thesis is the development of materials and architectures for all-polymer functional structures for photonic applications. The first part concerns the improvement and optimization of colorimetric and fluorescent sensing structures for the detection of various analytes in the vapor phase. Optical-readout sensors are portable and can provide an easy interpretation that needs no specialized training and can be visible to the naked eye. This makes them promising for applications in environmental control, health monitoring and food safety. The objective of the work was to investigate analyte diffusion processes into multilayered structures of polymer submicrometric films, and then optimizing the structure design and expanding the materials used in the field. First, sensors based on vapor diffusion in multilayered polymer dielectric mirrors with structural coloring were developed. Given their clear color change, this typology of sensors has been shown to be promising in the literature. However, as their response is limited by the diffusion speed of molecular species, they can suffer from slow detection of vapor-phase analytes. Next, I examine the use of fluorescent polymer films sensitive to microviscosity changes caused by exposure to volatile organic compounds and observing the changes in fluorescence during said exposure. The effect on the overall diffusion of capping layers deposited on top of the fluorescent polymer was investigated to quantify the effect of the barrier polymer on the selectivity of the sensor. Finally, I employed the solution processing protocols developed for novel low refractive index polymer suspensions that were initially utilized for the sensors to engineer structures for fluorescence control. When two highly reflecting structures encapsulate a luminescent material in a submicrometric space, this changes the photoluminescence properties in structures called optical microcavities. While the highly reflecting structures can be metallic mirrors, these have limited reflectance intensity, high absorbance losses, as well as a lack of tunability. Instead, the use of dielectric mirrors enables very high reflectance at desired wavelengths. In addition, the use of compliant polymer materials allows the future use of these structures to construct more efficient flexible devices. I was able to develop highly reflecting microcavities for emitters in the visible range as well as in the near infrared. Besides achieving high amplification of fluorescence intensity, I was also able to report for the first time a change in the radiative rate of the fluorescence for polymer structures. As these effects were so far only observed in planar structures of inorganic nature or more complex polymer three-dimensional systems, this presents a breakthrough in the field. In this introduction I will give a wide but deep overview of the optics of multilayered polymer films, their diffusion peculiarities, and use for sensing. Furthermore, I will address the topic of solid-state organic fluorophores and controlling their photoluminescence through engineering the dielectric environment. This will be followed by a chapter-by-chapter exploration of the results obtained during the doctoral training as adapted from already published or drafted work. Finally, the outlook and possible future implications and developments of this research will be examined

Directional Fluorescence Spectral Narrowing in All-Polymer Microcavities Doped with CdSe/CdS Dot-in-Rod Nanocrystals

We report on the fluorescence properties of high optical quality all-polymer planar microcavities embedding core 12shell dot-in-rod CdSe/CdS nanocrystals. Properly tuned microcavities allow a 10-fold sharpening of the nanocrystals fluorescence spectrum, resulting in a reduction of the bandwidth from 24 to 2.4 nm, which corresponds to a quality factor larger than 250. A 5-fold peak photoluminescence intensity enhancement is measured, while the overall number of emitted photons is reduced. Time-resolved photoluminescence and quantum yield for microcavities and suitable references show the presence of two decays related to differences in nanocrystal size distribution. The slower decay rate, which becomes faster when the nanocrystals are embedded into the microcavity, is assigned to longer nanorods with emission spectrally overlapped to the cavity mode. Conversely, the short-living component, which is assigned to an impurity of shorter nanorods, remains unaffected by the microcavity